1. Field of the Invention
The present invention relates to an image forming method for converting an electrostatic latent image into a visible image in electrophotography or electrostatic printing. More particularly, it relates to an image forming method, an image forming apparatus and an apparatus unit in electrophotography, having a step of contact charging of an electrostatic latent image bearing member (herein after "photosensitive member") by means of a charging member being externally applied with a voltage and in contact with the photosensitive member, and a step of electrostatic contact transfer of the developed image to a transfer medium by means of a transfer member externally being applied with a voltage and pressed against a transfer medium.
2. Related Background Art
For electrophotography, various methods are known as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-4748. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member having a photoconductive material, and subsequently developing the latent image with a toner by various means, transferring the toner image to a recording medium such as paper if necessary, and by fixing with heat, pressure, heat-and-pressure, or solvent vapor. The toner remained on the photosensitive member is cleaned by various means, and then the above process is repeated.
In recent years, such copying apparatus are not only used in office work for copying originals, but they have come into use as an information output machinery in combination with other information processing machines by the introduction of digital techniques, or as a multi-functional copying machine which can process and edit the image informations to prepare new originals, or as personal copying machines for private use.
Thus, the higher speed and higher image quality, and more down-sizing are strongly required for the apparatus, as well as the higher reliability.
In printers or copying machines using electrophotographic techniques, corona dischargers have been widely used, i) as a means to uniformly charge the surface of a photoconductor (an electrostatic latent image bearing member) and ii) as a means to transfer a developer-developed image from the surface of the photosensitive member. Recently, a new method has been studied and developed to practical use in charging or image transfer processes, in which direct charging or image transfer is carried out by bringing in contact or pressing a charging member to the surface of the photosensitive member while externally applying a voltage to the charging member.
Such a method is disclosed, for example, in Japanese Patent Applications Laid-open No. 63-149669 and No. 2-123385. These are concerned with contact charging or contact transfer process, wherein an photosensitive member is uniformly charged by pressing a conductive elastic roller to which a voltage is applied externally. The uniformly charged photosensitive member is then exposed to form thereon an electrostatic latent image, end after the developement process, another conductive elastic roller with an applied voltage is pressed to the developed image bearing member with a transfer medium passing between to transfer the developed image to the transfer medium, followed by fixation to obtain a copied image.
An example of the image forming method having such a contact charging system and a contact transfer system is explained below with reference to a schematic illustration in FIG. 4.
Reference numeral 101 denotes a rotating drum type image bearing member (photosensitive member). The photosensitive member 101 is basically comprised of a conductive substrate layer 101b made of aluminum or the like and a photoconductive layer 101a formed on it, and it clockwise rotates as viewed in the drawing, at a given peripheral speed (process speed).
Reference numeral 102 denotes a charging roller, which is basically comprised of a mandrel at the center and a conductive elastic layer formed on its periphery. The charging roller 102 is in contact with the surface of the photosensitive member 101 at a given pressure, and rotates following the rotating photosensitive member 101. Reference numeral 103 denotes a charging bias power supply from which a voltage is applied to the charging roller 102. Application of a bias to the charging roller 102 charges the surface of the photosensitive member 101 to a given polarity and potential. Subsequent imagewise exposure 104 forms electrostatic latent images, which are successively converted into visible toner images through a developing means 105.
Reference numeral 106 denotes a transfer roller, which is basically comprised of a mandrel 106b at the center and a conductive elastic layer 106a formed on its periphery. The transfer roller 106 is in contact with the surface of the photosensitive member 101 at a given pressure, and rotates at a speed equal to, or different from, the peripheral speed of the photosensitive member 101. As a transfer medium 108 is transported between the photosensitive member 101 and the transfer roller 106, a bias with a polarity reverse to that of the toner is applied from a transfer bias power supply 107 to the transfer roller 106, so that the toner image on the photosensitive member 101 is transferred to the surface of the transfer medium 108.
Subsequently, the transfer medium 108 is transported to a fixing assembly 111 basically comprised of a fixing roller 111a internally provided with a halogen heater, and an elastic-material pressure roller 111b which is in contact with 111a at a given pressure, and while the transfer medium 108 is passing between the rollers 111a and 111b, the toner image is fixed on the transfer medium 108 and output as a printed image.
After the transfer of the toner image, adherants such as untransferred toner are removed from the surface of the photosensitive member 101 to clean the surface by means of a cleaning assembly 109 provided with an elastic cleaning blade which is in pressure contact with the photosensitive member 101 in the counter direction. The surface is then erased through a charge eliminating exposure assembly 110, and is repeatedly used for image formation.
Such a contact charging and contact transfer system can achieve uniform charging of a photosensitive member and satisfactory image transfer therefrom at a bias of a lower voltage compared with corona charging and corona transfer, and is advantageous in down sizing of chargers and in preventing the generation of corona discharge by-products such as ozone.
In the image forming apparatus as described above, however, the external surface of the photosensitive member is abraded by the cleaning blade or by a developer as the image formation is repeated. Thus the charge performance may change along the change, in equivalent capacity due to the decrease in the thickness (layer thickness or film thickness) of the photosensitive member.
When image formation is repeated and the film thickness of the photosensitive member decreases, the direct currents flowing to the charging roller increases and the surface potential on the external surface of the photosensitive member rises. The decrease in the film thickness of the photosensitive member and the rise of the surface potential cause an increase in development contrast which increases the developed image density and at the same time gives insufficient reverse contrast to the potential of a white image, resulting in "fogged" images because the white apes is thinly developed with the developer.
Thus the poop mechanical durability such as thinning of the photosensitive member greatly affects the lifetime of the photosensitive member.
To cope with this problem, an organic photosensitive member having a surface layer containing a lubricant such as a fine fluorine resin powder is proposed in Japanese Patent Application Laid-open No. 63-30850 to reduce the wear of the surface layer of the photosensitive member and prevent deterioration of its charge performance.
As for the transfer roller, it is known that the relationship between the voltage applied thereto and the current flowing therethrough, will greatly varies depending on the environmental factors. Moreover, ususally any size of the transfer medium can be used in the apparatus of this type provided it is not larger than the maximum-size transfer medium. Therefore, there becomes the area where the transfer medium is not present and the photosensitive member comes into direct contact with the transfer roller when a small-size transfer medium is used, which causes uneven facility of current flow inviting voltage drop.
That is, in the contact transfer system, it is difficult to secure good transfer performance for all sorts of transfer mediums in all environmental conditions by either constant voltage control or constant current control. Accordingly, when transfer efficiency lowers, it becomes impossible to meet various needs for copying materials including post cards, OHP films, etc. which are unfavorable for the transfer. The "transfer hollow" phenomenon (blank areas due to poor transfer), which is a kind of faulty transfer, may also occur at the area where the developer tends to gather (an edge development area), e.g., the area corresponding to the contour or line of an image. This is considered due to the developer laid in a larger quantity on the edge development area than other usual areas to tend to cause agglomeration of the developer, resulting in a lowering of response to a transfer electric field. Thus, this system has the problem that it is difficult to obtain high-quality images faithful to latent images.
To overcome this problem, some improvements have been achieved in the control methods proposed in Japanese Patent Application Laid-open No. 63-276106 and No. 2-264278, but is still unsatisfactory.
In relation to the step of fixing toner images to transfer mediums such as transfer paper, the most prevalent method at present is heat-pressure system using a heating roller, i.e., heat-roll fixing system. In the heat-roll fixing system, waiting time for the heating roller to reach a given temperature is required, which is shortened as the speed of the apparatus is increased. Accordingly, faulty fixing tends to occur because of the instant fixation or the temperature drop of-the heating roller due to the pass of the transfer medium. Moreover, since the heating roller surface comes into contact with melted toner images, the toner images may partly adhere to the surface of a fixing roller, which may be further transferred to the subsequent transfer medium, causing so-called offset phenomenon.
For this reason, improvements in fixing performance of the developer have been sought, and a proposal is made in Japanese Patent Publication No. 63-32182 for the purpose of improving a low-temperature fixing performance of toners. In that publication, a toner containing as a binder resin component a vinyl polymer having at least one distribution peak in each of specific low-molecular weight and high-molecular weight regins is proposed. The toner contains the low-molecular weight component in a relatively large amount so that the low-temperature fixing performance can be improved. Further, Japanese Patent Application Laid-open No. 2-235069, disclosed a magnetic toner containing a binder resin having two peaks in its molecular weight distribution, in which, each dispersion ratio of the low-molecular-weight region and the high-molecular-weight region from the minimal point between two peaks as a reference point, is made small, to improve low-temperature fixing performance and anti-offset properties.
Although the toner as described above is improved in fixing performance, use of such a magnetic toner in an image forming apparatus having the contact charging system and the contact transfer system tends to cause the transfer hollow. This is because a solvent used in the solution polymerization of the resin remains in the magnetic toner in a large quantity, accelerating the agglomeration of toner because of the pressure applied in the transfer step.
Although one may contemplate to produce vinyl resins by polymerization other than the solution polymerization, as is stated in Japanese Patent Application Laid-open No. 2-272459, the solution polymerization is considered essential as a polymerization process for vinyl resins to obtain a sufficient (low-temperature) fixing performance.
Organic photosensitive members (organic photoconductors) are prevalently used in medium-speed machines in order to reduce the size and the cost of the machine. In particular, Japanese Patent Application Laid-open No. 63-30850 discloses, as previously stated, an organic photosensitive member having a surface layer containing a lubricant such as a fine fluorine resin powder for the purpose of decreasing the wear of the surface layer of an organic photosensitive member to prevent deterioration of its charge performance. The organic photosensitive member containing such a fine fluorine resin powder certainly has a longer lifetime itself. However, when a magnetic toner containing a binder resin synthesized from vinyl monomers in the presence of an aromatic organic solvent suitable for solution polymerization is used for image development, the aromatic organic solvent remaining in the magnetic toner in a large quantity may contaminate the surface resin of the organic photosensitive member, so that the resin component of the magnetic toner tends to adhere to the surface of the photosensitive member, tending to cause filming. Moreover, when the surface layer of the photosensitive member contains the fine fluorine resin powder, the surface of the photosensitive member is not much abraded by a cleaning blade or the magnetic toner, resulting in the accumulation of the resin component of the magnetic toner on the surface of the photosensitive member in a large quantity, that is, more filming tends to occur.